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SILICON CHIP MIDI DRUM KIT by JOHN CLARKE PART 3 – building the sensors and stand As mentioned last month, there are two basic pad sensor types: those based around a piezo transducer and those based around the optical sensing circuitry. Which one you use is up to you. The piezo-based pads require tapping with drum sticks or similar items in the conventional “drum” manner. The optical pads can be tapped by hand, making them easier to use. We also use an optical pickup for foot-activated sensors. This is because the system is more robust. A piezo transducer that can be attacked with a foot pedal may well be destroyed rather quickly. Each pad type is described in the separate sections following. Building The Optical PC Board Circuit details were shown in Part 1 but are repeated here for convenience along with the PC board overlay for the optical sensor (there is no PC board for the piezo sensor). The optical sensor PC board is coded 01211053 and measures 52 x 31mm. Once again, before assembly check the underside for any possible shorts between tracks or breaks in the copper. As you did for the other boards, install the resistors and diode (as shown), the PC stakes and the capacitor (which is mounted on its side to allow clearance above the PC board). Finally, the optical sensor is mounted directly onto the PC board, making sure it is oriented correctly. 86  Silicon Chip siliconchip.com.au The Simplest Piezo Sensor The simple piezo sensor is based around a length of 20mm (27mm OD) PVC water pipe. The piezo transducer attaches to the end of the pipe with the shielded cable connected to the transducer and the other end to an RCA plug. To use, the pipe is held in the hand at the opposite end to the piezo and the pipe is then tapped with a stick. Note that the two red and black wires that are supplied connected to the piezo transducer need to be unsoldered and Parts List – Piezo Sensor 1 piezo transducer 1 300mm length of 20mm PVC water pipe (27mm OD) 1 2m length of single- cored shielded cable 1 RCA line plug 2 No.4 self-tapping screws Percussive (Piezo) Sensor Pad This sensor pad needs to be mounted onto a frame using 25mm tubing (see separate diagram). The sensor is played with a drumstick or similar item. Note that the front edge is shown curved to ensure that there is a consistent response from the sensor when the pad is tapped around its circumference. Do not be tempted to use a rectangular edge. This will give dead spots in several positions on the pad where the sensor will not detect a strike on the plate. In addition, do not use metal in preference to the MDF since this will resonate for too long after it is struck. The sensor pad is constructed as shown at right. Cut out the circular section of the MDF panel with a jigsaw and paint the pad with gloss black paint. The piezo replaced with the shielded wire. You must carefully prise off the back of the piezo case to gain access to the wiring. transducer needs to be rewired to have the shielded cable soldered to its piezo and brass sections. The rear of the piezo housing can be carefully prised off to gain access to this wiring. We used an epoxy resin smeared around the edge of the back of the piezo housing before securing with the M2.5 screws into the MDF. This ensures a good physical contact with the MDF plate. The top of the MDF sensor plate can be painted black or covered with a thin sheet of vinyl using contact adhesive. The vinyl can deaden the sound produced by striking it with a stick. Alternatively, the stick can be rubberised. In most cases, the sound of the stick directly onto the painted MDF will not cause any concern. Parts List – Piezo Pad 1 piezo transducer 1 140 x 140mm piece of 3mm MDF 1 120mm length of 20mm PVC water pipe (27mm OD) 2 25mm cable cleats (Farnell NP10 pro-power Cat 725-7399) 2 M4 x 12mm screws 4 M4 nuts 1 ¼” x 4” bolt and nut 1 6.35mm ID x 9mm OD x 22mm long plastic spacer (electrical lamp fitting) 2 M2.5 x 4mm long screws Miscellaneous Gloss black paint, vinyl (optional), epoxy resin, contact adhesive This upsidedown view of a piezo sensor pad is mainly to show the method of mounting the transducer. It also shows the method of mounting to the stand via a clamp and length of PVC pipe (also see diagram below). siliconchip.com.au January 2006  87 Bongo Drums The bongo drums are made using 12mm MDF and 18g (1mm) aluminium sheeting. The MDF makes up the top and bottom dual semi-circular sections while the aluminium sheet is wrapped around the sides of the MDF and secured in place with 6g x 20mm countersunk wood screws. Cutouts in the top are blocked off using MDF timber glued on the underside. These provide the mounting base for the optical sensor PC board to be secured using the 4g self-tapping screws. The MDF cut-outs and the semicircular shape can be cut using a jig saw. Glue the timber to the underside of the top panel cutouts and paint the timber. Wiring the PC boards involves following the overlay diagram for the optical pad sensor and wiring. Note that only two DC sockets are necessary – the second PC board derives its power from the first using hookup wire between the + and + power terminals and the - and - terminals. The DC sockets mount on the aluminium sides of the drums. The signal leads pass through the aluminium via a rubber grommet. A cable tie holding the two signal leads behind the grommet will help prevent the leads pulling out. An alternative arrangement is to attach two RCA panel sockets to the drum and use RCA-to-RCA leads for the signal connections. Make up a DC socket to DC socket lead and terminate the signal leads with‑ RCA plugs. The pressure plate is secured to the top of the drum using 4g screws. The screw that passes through into the optical sensor must be exactly located so that it fits neatly into the optical slot. The plate is bent as shown. Adjustment The slight upward bend allows the screw to have clearance inside the optical slot. This screw needs to be adjusted carefully so that there is a change in voltage at the signal output when the plate is moved. The completed bongo drums. In the photo below left the drums are partially disassembled, showing the optical sensors and plates. The easiest way to adjust this is to firstly make up an RCA socket to RCA plug test connector as shown below and insert this in line between the signal output lead and the MIDI Drum Kit. Alternatively, access the rear of the RCA socket inside the MIDI Drum Kit. Plug into one of the RCA input sockets for the MIDI Drum Kit. Apply power to the drum kit and also connect the DC power lead between the drum kit’s DC output socket and one of the DC input sockets on the bongo drums. Connect a multimeter set to read DC volts to the test connector. The voltage reading should be either small (< 0.1V) if the screw is too far out of the sensor gap or up to 5V is the screw is too far into the slot. Adjust the plate angle and screw setting so that the normal voltage is very close to 200mV. This voltage should rise to close to 5V when the plate is pressed. Ensure the plate is free to move without scraping the sides of the MDF and without the screw scraping the inside of the optical sensor. With the arrangement shown having a nut on top and on the bottom of the plate, the screw can be turned in or out to make the adjustment. An alternative method is to cut the screw to length so that it does not need adjusting. If you have cut too much from the screw, the length can be built up with some solder. This makes it easy to file down so that the length is just right when the plate is secured down with the screws into the MDF panel. Finishing We finished the drums by covering with loudspeaker carpet. This is attached with contact adhesive. The covering is optional. This easy-to-make jig will allow you to adjust the Bongo Drums (and other optical sensors) very easily, by connecting to your multimeter and adjusting the screw to give maximum voltage change. 88  Silicon Chip siliconchip.com.au Parts List – Bongo Drums Plan and elevation of the bongo drums. Top right shows how the pressure plate fixes onto the surface of the drums, with the PC board underneath in the “well”. Speaking of the well, here’s how to make it. This diagram is half the size of the finished drum – so if you have an enlarging photocopier, 200% is the magic number! siliconchip.com.au 2 PC boards coded 01211053, 52 x 31mm 2 photo-interrupters 2 1N4004 diodes 2 470mF 16V PC electrolytic capacitors 4 470W 0.25W 1% resistors 2 panel-mount DC sockets 2 line DC plugs 8 PC stakes 1 50mm long cable tie 4 metres single core shielded cable 2 RCA line plugs 2 135 x 245mm pieces of 12mm MDF 2 30 x 220mm pieces of 12mm MDF 2 120 x 707mm sheets of 18g (1mm) aluminium 2 61 x 108mm sheets of 18g (1mm) aluminium 2 20 x 58mm sheets of 18g (1mm) aluminium 8 4g x 12mm self-tapping screws 12 6g x 20mm csk wood screws 1 M3 x 12mm screw 1 3mm star washer 2 M3 nuts 1 6mm ID rubber grommet Miscellaneous Black paint, contact adhesive, speaker carpet (optional), PVA glue January 2006  89 Optical Sensor In many ways, the optical sensor is similar to the Bongo drums in construction. Most of the construction for this applies to the optical sensor. The sensor is designed to mount onto a frame described on the next page. When used as a foot controller it can be placed on the floor rung of the stand. The current design is not suitable for use on the higher rungs of the stand. This is because there will be too much stand movement when the sensor is pressed. A heavy-duty stand made from tubular steel could be used to support several optical pad sensors. The optical pad sensors would also need to be held in position with steel clamps instead of plastic ones. In addition, use two DC sockets for the power input and output as shown in the main optical sensor wiring. The second DC socket allows for interconnecting power between sensors. The 12mm MDF will require trimming back in thickness by 3mm where the two plastic clamps attach at the rear of the pad. Use a chisel to do this. The reduction in thickness is so that the clamp will close up sufficiently to mount tightly onto the 25mm frame. The MDF material for the sensor is painted using gloss black paint before assembly. A rubber stick-on foot is placed at the front edge of the cut-out to provide a stopper for the pressure plate. The sensor PC board is attached using M3 screws and nuts. You may need to use some Nylon washers to space the PC board slightly above the 3mm MDF baseplate. Note that the pressure plate needs the centre screw to be adjusted correctly as described in the bongo drum section. PIC T/C Note These optical pads are not ideal for hitting with a drumstick since it sets up resonance in the aluminium plate. This causes the drum sound to be played more than once for each strike. If you need to use the optical pads with sticks, then the setting-up procedure where the DC voltage is set to around 200mV needs to be changed. The initial voltage is set to the saturation voltage of the output transistor in the optical sensor. This is about 100mV and is observed to be the voltage that does not change until the pressure plate is moved by a millimetre or two. Best adjustment is found by trial and error, but the plate needs to be set so that it needs some movement before any voltage change is found at the sensor output. Parts List 1 PC board coded 01211053, 52 x 31mm 1 Photo interrupter 1 1N4004 diode 1 470mF 16V PC electrolytic capacitor 2 470W 1/4W 1% resistors 1 line DC plug 4 PC stakes 1 2-metre length of single core shielded cable 1 RCA line plug 1 140 x 142mm piece of 12mm MDF 1 140 x 120mm piece of 3mm MDF 1 61 x 108mm sheet of 18g (1mm) aluminium 1 20 x 58mm sheet of 18g (1mm) aluminium 2 25mm cable cleats (Farnell NP10 pro-power Cat 725-7399) 4 M3 x 12mm screws and M3 nuts 4 4g x 12mm self tapping screws 1 M3 x 12mm screw 1 3mm star washer 2 M3 nuts 2 4” x ¼” bolts 2 ¼” nuts 4 3mm Nylon washers 2 50mm cable ties Again reproduced half size, this diagram shows how the optical sensor unit is constructed. 90  Silicon Chip Miscellaneous Black paint, PVA glue siliconchip.com.au Building A Playing Stand One or two sensors alone are fine sitting on a bench, etc, (or even the floor!) but when a number of sensors are used, some form of frame or stand is necessary so they can mount in their correct positions for playing. We made up a stand using conventional hardware components. Broomsticks were used instead of dowell because they’re usually a lot cheaper! The stand was glued together with builders’ adhesive for the Tee joins and PVA glue for the MDF bracket to the 25mm rod. It was painted in a gloss black. Note that the design allows the top rung to be removed. This is for easy transport of the frame on the back seat of a car. The broomstick pieces are cut to length, noting that the stick is passed right through the T-piece rather than cutting it at this point. Each T-junction is made stronger by making a 25mm hole part way into the side of the straight through broomstick. Note that the Vinidex T-pieces allow the broomstick to pass easily through it. Other brands may need drilling or filing out to allow the stick to enter right through the piece. The MDF right angle bracket is included to give extra support in the forward and backward direction. The wires for the sensors can be made tidy along the frame using cable clips. The percussive sensor is designed to be able to be adjusted to any angle, making it very suited for stand mounting. However, if using the optical sensor on the stand, it must be mounted on the floor rung of the frame, so the front edge of the pad rests on the floor to add extra support. As mentioned, the stand and optical sensor pad mounting must be of heavier construction than the one shown here if you intend to use optical pad sensors throughout. Parts List 6 broom sticks, 25mm in diameter 6 20mm PVC water pipe Tee pieces (Vinidex) 2 20mm PVC water pipe joiners 1 150 x 150mm x 12mm MDF sheet 4 25mm rubber pipe ends Miscellaneous Black gloss paint, PVA glue, builders adhesive This is the stand we put together from bedknobs and broomsticks (OK, not too many bedknobs). It’s just one possible arrangement for mounting the controller and various sensors, giving room for foot pedals underneath. siliconchip.com.au January 2006  91 Foot Controller The foot controller is generally used to control a bass drum sound. We show how to build a foot controller for this purpose using an optical sensor. An alternative foot controller is shown under the Optical pad heading. The optical pad is made into a plastic box and secured to the upright MDF piece with M4 screws and nuts. This also sandwiches the pressure plate in between the box and MDF. The PC board for the optical pickup is supported on M3 tapped standoffs (9mm long) and held using M3 x 10mm screws. The screw mounted in the pressure plate to interrupt the optical sensor must be adjusted as described in the Bongo Drums set-up. The DC sockets are mounted at each end of the box. The signal output is wired to the 6.35mm jack socket that is mounted on an aluminium bracket. You will need to make up a 6.35mm jack plug to RCA plug lead for the connection between the foot controller and the MIDI Drum Kit. The footswitch is made using the MDF panelling as shown. We painted the assembly with black paint and covered the foot areas with speaker carpet. If you want to build your own foot controller, here is how to do it. The alternative is to use the information in this diagram to modify a commercial foot pedal. 92  Silicon Chip siliconchip.com.au Footswitch The footswitch is a commercially made unit – all it really needs is to have an RCA plug wired to its end. However, the wiring supplied with the switch is a little too thick for the RCA plug so is best replaced with a single core shielded cable instead. Connect the wire between the normally open and common connections on the microswitch. Parts List 1 PC board coded 01211053, 52 x 31mm 1 bulkhead plastic box, 65 x 38 x 25mm 1 Photo interrupter 1 1N4004 diode 1 470mF 16V PC electrolytic capacitor 2 470W 1/4W 1% resistors 1 6.35mm mono jack socket 2 line DC plugs 4 PC stakes 1 2-metre length of single core shielded cable 1 6.35mm mono plug 1 RCA line plug 1 65 x 120mm length of 1mm aluminium 1 20 x 40mm length of 1mm aluminium 1 60 x 20mm 1mm aluminium 1 100 x 35mm sheet of 1mm aluminium 1 85mm but hinge 6 8g csk wood screws cut to 12mm 7 6g csk wood screws cut to 12mm 1 5g x 20mm screw 1 M3 x 15mm screw 1 3mm star washer 2 M3 nuts 2 M4 x 15mm screws 2 M4 nuts 4 M3 tapped x 9mm spacers 8 M3 x 6 screws 1 20mm long by 6mm OD compression spring (from clothes peg) 3 15mm OD x 6mm ID rubber grommets 1 100 x 320mm length of 12mm MDF 1 100 x 80mm length of 12mm MDF 1 100 x 220mm length of 12mm MDF 1 65 x 47mm length of 12mm MDF Parts List 1 footswitch (eg, Jaycar SP-0760) 1 1m length of single core shielded wire 1 RCA line plug NEXT MONTH: The final part – software – and how to drive it! Miscellaneous Black paint, PVA glue, speaker carpet, contact adhesive siliconchip.com.au January 2006  93